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ADD: MuPDF v1.26.7: the MuPDF source as downloaded by a default build of PyMuPDF 1.26.4. The directory name has changed: no version number in the expanded directory now.
author Franz Glasner <fzglas.hg@dom66.de>
date Mon, 15 Sep 2025 11:43:07 +0200
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23 .TH libcurl-thread 3 "13 Jul 2015" "libcurl" "libcurl thread safety"
24 .SH NAME
25 libcurl-thread \- libcurl thread safety
26 .SH "Multi-threading with libcurl"
27 libcurl is thread safe but has no internal thread synchronization. You may have
28 to provide your own locking should you meet any of the thread safety exceptions
29 below.
30
31 \fBHandles.\fP You must \fBnever\fP share the same handle in multiple threads.
32 You can pass the handles around among threads, but you must never use a single
33 handle from more than one thread at any given time.
34
35 \fBShared objects.\fP You can share certain data between multiple handles by
36 using the share interface but you must provide your own locking and set
37 \fIcurl_share_setopt(3)\fP CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.
38 .SH TLS
39 If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are
40 then of course using the underlying SSL library multi-threaded and those libs
41 might have their own requirements on this issue. You may need to provide one
42 or two functions to allow it to function properly:
43 .IP OpenSSL
44 OpenSSL 1.1.0+ "can be safely used in multi-threaded applications provided that
45 support for the underlying OS threading API is built-in." In that case the
46 engine is used by libcurl in a way that is fully thread-safe.
47
48 https://www.openssl.org/docs/man1.1.0/man3/CRYPTO_THREAD_run_once.html#DESCRIPTION
49
50 OpenSSL <= 1.0.2 the user must set callbacks.
51
52 https://www.openssl.org/docs/man1.0.2/man3/CRYPTO_set_locking_callback.html#DESCRIPTION
53
54 https://curl.haxx.se/libcurl/c/opensslthreadlock.html
55
56 .IP GnuTLS
57 https://gnutls.org/manual/html_node/Thread-safety.html
58 .IP NSS
59 thread-safe already without anything required.
60 .IP Secure-Transport
61 The engine is used by libcurl in a way that is fully thread-safe.
62 .IP WinSSL
63 The engine is used by libcurl in a way that is fully thread-safe.
64 .IP wolfSSL
65 The engine is used by libcurl in a way that is fully thread-safe.
66 .IP BoringSSL
67 The engine is used by libcurl in a way that is fully thread-safe.
68 .SH "Other areas of caution"
69 .IP Signals
70 Signals are used for timing out name resolves (during DNS lookup) - when built
71 without using either the c-ares or threaded resolver backends. When using
72 multiple threads you should set the \fICURLOPT_NOSIGNAL(3)\fP option to 1L for
73 all handles. Everything will or might work fine except that timeouts are not
74 honored during the DNS lookup - which you can work around by building libcurl
75 with c-ares or threaded-resolver support. c-ares is a library that provides
76 asynchronous name resolves. On some platforms, libcurl simply will not
77 function properly multi-threaded unless the \fICURLOPT_NOSIGNAL(3)\fP option
78 is set.
79
80 When \fICURLOPT_NOSIGNAL(3)\fP is set to 1L, your application needs to deal
81 with the risk of a SIGPIPE (that at least the OpenSSL backend can
82 trigger). Note that setting \fICURLOPT_NOSIGNAL(3)\fP to 0L will not work in a
83 threaded situation as there will be race where libcurl risks restoring the
84 former signal handler while another thread should still ignore it.
85 .IP "Name resolving"
86 \fBgethostby* functions and other system calls.\fP These functions, provided
87 by your operating system, must be thread safe. It is very important that
88 libcurl can find and use thread safe versions of these and other system calls,
89 as otherwise it can't function fully thread safe. Some operating systems are
90 known to have faulty thread implementations. We have previously received
91 problem reports on *BSD (at least in the past, they may be working fine these
92 days). Some operating systems that are known to have solid and working thread
93 support are Linux, Solaris and Windows.
94 .IP "curl_global_* functions"
95 These functions are not thread safe. If you are using libcurl with multiple
96 threads it is especially important that before use you call
97 \fIcurl_global_init(3)\fP or \fIcurl_global_init_mem(3)\fP to explicitly
98 initialize the library and its dependents, rather than rely on the "lazy"
99 fail-safe initialization that takes place the first time
100 \fIcurl_easy_init(3)\fP is called. For an in-depth explanation refer to
101 \fIlibcurl(3)\fP section \fBGLOBAL CONSTANTS\fP.
102 .IP "Memory functions"
103 These functions, provided either by your operating system or your own
104 replacements, must be thread safe. You can use \fIcurl_global_init_mem(3)\fP
105 to set your own replacement memory functions.
106 .IP "Non-safe functions"
107 \fICURLOPT_DNS_USE_GLOBAL_CACHE(3)\fP is not thread-safe.